Circuit for automatic operation of a series brake upon power loss during a regenerative braking period

ABSTRACT

An anti-regeneration circuit for preventing a regenerated current produced by an overhauling DC motor from setting a series brake. The anti-regeneration circuit includes a current blocking device 90 which blocks the flow of regenerated current in the crane control circuit 10 and a surge suppresser 94 for protecting the current blocking device 90 from a high inverse peak voltage produced by de-excitation of a crane control circuit 10 upon the sudden loss of power.

FIELD OF THE INVENTION

The present invention relates to crane operating circuits and inparticular to an automatic anti-regeneration circuit which preventsregenerated DC current produced by an overhauling DC lifting motorduring a power outage from slowing or preventing the setting of a seriesbrake.

BACKGROUND OF THE INVENTION

Cranes have been an important part of industry for many years. Cranesdriven by DC electric motors are used to hoist and move heavy loads fromone location to another within the crane's service area. Since theseloads can be extremely heavy, and can include molten metals in the ironand steel industries, it can easily be seen that an automatic brakingsystem to stop the lowering of the load during a power outage isimportant. Two types of brakes, electrical and mechanical, are commonlyused with electric motors. Electrical brakes are generally used whenthere is no tendency for rotation of the motor due to a heavy load.Dynamic braking is a common method of electrical braking used in DCmotors. In dynamic braking, when power to motor circuit is removed, themotor will continue to rotate due to its momentum and will generate acounter electromotive force (CEMF) as long as it remains in rotation.Since the polarity of the CEMF is opposite to that of the voltage fromthe DC power supply, current flow in the armature will also be in theopposite direction. This reverse current flow in the motor circuitcauses a torque opposite to the normal motor rotation to be developed,thus causing the motor slow. In dynamic braking, the speed at which themotor slows can be controlled by selectively changing the electricalresistance in either the field circuit, or the armature circuit of themotor. The most effective control is a combination of the two. In thefield circuit, an acceleration resistor comprised of a bank of one ormore resistors connected in series with the field coil are used toselectively weaken or strengthen the magnetic field through which thearmature rotates. Low resistance in the field circuit produces a strongmagnetic field and increases the CEMF produced while a high resistancein the field circuit weakens the magnetic field and decreases the CEMFproduced. In the armature circuit, a dynamic braking resistor comprisingone or more load resistors connected in the dynamic braking loopselectively controls current flow in the armature. Low resistance in thedynamic braking loop permits a high current flow in the armature andreduces the CEMF produced while a high resistance in the dynamic brakingloop reduces current flow in the armature and increases the CEMFproduced. The time required to bring the motor to a complete stop willdepend on the resistance values of the acceleration resistor and thedynamic braking resistor in the motor circuit, the friction of thesystem, and the external load on the motor (weight of the load supportedby a crane lifting motor). Dynamic braking is most effective in shunt orlightly compounded motors since the field is in parallel with thearmature and therefore independent of armature current. During thelowering operation of a DC crane, the series wound DC motor isselectively manipulated into a shunt connected machine (motor, armatureand field are connected in parallel) through the use of contactscontrolled by a master switch in the crane control circuit. Configuringthe motor as a shunt machine allows the motor to take advantage ofdynamic braking when lowering a load. Mechanical brakes are generallyspring-set brakes and are normally engaged when power to the motor isnot present. When power is applied to the motor, the brake is releasedby means of a solenoid-operated mechanism that overcomes the force ofthe engagement spring. The solenoid is operated by a coil electricallyconnected in series with the motor, such that, when power is applied tothe motor, the solenoid coil will be activated, thereby releasing themechanical brake. When power is removed from the motor circuit, whetherby normal crane operation or by a power outage, the solenoid coil isdeactivated, thereby activating the spring-set mechanical brake. Duringnormal crane operation, the master switch controls the direction andspeed of the DC lifting motor by operating contacts in the motorcircuit. When the master switch is moved to the OFF position, dynamicbraking will quickly slow the motor to a stop. This will stop thegeneration of the CEMF and the flow of reverse current in the motorcircuit thus deactivating the series-wound solenoid and activating thespring-set mechanical brake. However, if the crane is lowering a heavyload at some speed other than the lowest speed when power is lost, theweight of the load will cause the motor to continue rotating and therebycontinue generating a CEMF. The reverse current in the motor circuitwill prevent the setting of the mechanical brake by the series-woundsolenoid. If the load is heavy enough to maintain the motor in anoverhauling state, the dynamic brake can not slow the motor to a stopand the mechanical brake will not be set. This will result in thecontinued lowering of the load until it reaches the floor or othersupporting means.

In an effort to overcome this problem, crane control systems have beenprovided with emergency stop switches and "dead man" switches in thecrane operating circuitry. These switches would operate or control acontact in either the undervoltage circuit or the armature circuit,which would cause the armature circuit to be opened, thereby stoppingthe flow of current. The problem with these switches is that theyrequire some action by the crane operator to initiate activation. Duringan emergency, this operator required action could be difficult orimpossible. It is also possible for the switch operation to be defeatedor rendered inoperable by the operator. This has been particularly truewith respect to the "dead man" switch, which is generally a springbiased normally open switch requiring the operator to continuously holdit in the activated position while lowering the crane's load. A morerecent method, as described in a paper by M. A. Urbassik entitled"Automatic Brake Setting During DCCP Regenerative Hoist Control PowerLoss Condition", presented at the 1997 A.I.S.E. proceedings, employs alow voltage monitoring relay (LVMR) which monitors the DC bus voltage.The LVMR, upon sensing a change in the DC bus voltage, initiates theactivation of the series brake. Although it is not disclosed how theseries brake activation is initiated, it would be obvious to open acontactor in the undervoltage circuit of the crane control circuit orthe motor circuit as in the "dead man" switch or E-Stop button. Thisparticular application, as further described in the paper, requires someadjustment of the LVMR, depending on the characteristics of theparticular crane system on which it is to be installed. It wouldtherefore be desirable to have an automatic anti-regeneration circuitwhich can easily be connected to an existing crane control system, andwhich does not require any additional electrical adjustments for properoperation with the existing crane control system in which it is to beinstalled. It would also be desirable to have a completely electronicapplication of the automatic anti-regeneration circuit, thus eliminatingmechanical elements such as contacts, which can have mechanicalfailures.

SUMMARY OF THE INVENTION

The present invention provides a simple anti-regeneration circuit whichcan be provided in a factory assembled crane control circuit, or whichcan be provided as an easily installable kit for retrofitting existingcrane motor circuits. The components of each retrofit kit are selectedfor a particular range of NEMA motor ratings such that no additionalcalibration or fine-tuning of the anti-regeneration circuit is requiredafter installation. The anti-regeneration circuit prevents DC currentproduced by a DC crane lifting motor incurring a power outage during alowering operation from prohibiting the activation of a series brake.The anti-regeneration circuit includes a power diode and a surgesuppresser. The power diode is electrically connected in series with thecrane lowering contactor and is connected to permit normal current flowin the motor circuit and prohibit reverse current generated by theoverhauling motor from energizing the crane control circuit. The surgesuppresser includes a control diode and a discharge resistor,electrically connected in series with one another, and together,electrically connected in parallel with the crane control circuit. Thecontrol diode is connected such that the discharge resistor iselectrically active only when the motor circuit loses power.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a typical crane operating circuit of theprior art configured in a normal fourth speed lowering mode with powersupplied by the DC power source.

FIG. 2 is a circuit diagram of a typical crane operating circuit of theprior art configured in a normal fourth speed lowering mode with a poweroutage.

FIG. 3 is a circuit diagram of a crane operating circuit of the priorart configured in a normal fourth speed lowering mode and employing aLVMR and a contact in the crane control circuit.

FIG. 4 is a circuit diagram of a crane operating circuit in accordancewith the present invention configured in a normal fourth speed loweringmode with power supplied by the DC power source.

FIG. 5 is a circuit diagram illustrating current flow in a craneoperating circuit assembled in accordance with the present inventionconfigured in a normal fourth speed lowering mode with a power outage.

Before one embodiment of the invention is explained in detail, it is tobe understood that the invention is not limited in its application tothe details of construction described herein or as illustrated in thedrawings. The invention is capable of other embodiments and of beingpracticed or being carried out in various other ways. Further, it is tobe understood that the phraseology and terminology used herein is forthe purpose of description and should not be regarded as limiting.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates a crane operating circuit, generally indicated byreference numeral 12, consisting of a hoist motor circuit, generallyindicated by reference numeral 10, and those elements of a crane controlcircuit, generally indicated by reference numeral 14, which are relevantto the following discussion of the present invention, both circuits 10and 14 being typical of the prior art. In FIG. 1, the arrows indicatethe direction of current flow during a normal load lowering operation atspeed 4. During the lowering operation, the series DC motor is operatingto oppose the downward pulling weight of the load and thereby permit acontrolled decent of the load. A DC power source 18 provides operatingpower for both circuits 10 and 14. In the motor circuit 10, currentpasses through a lowering contact 22, a motor armature 26, a seriesfield 30 (the motor), and a series-wound solenoid coil 34. Current flowin the series-wound solenoid coil 34 keeps a mechanical series brakefrom being engaged while the crane hoist motor is being operated. In thecrane control circuit 14, an undervoltage (UV) relay 38 monitors thevoltage level and is intended to open an undervoltage (UV) contact 42 inthe event of a loss of power from the power supply 18, therebyinterrupting current flow in both the control circuit 14 and the motorcircuit 10. A master switch 46 in the control circuit 14 controls thelowering or hoisting state of the crane by selectively opening orclosing the lowering contact 22 and hoisting contact 50 in the motorcircuit 10. The master switch 46 also controls the lowering and hoistingspeed by selecting resistance values for a dynamic braking resistor 54and an acceleration resistor 58. Selectively opening and closingcontacts 62 associated with the dynamic braking resistor 54 and contacts66 associated with the acceleration resistor 58 accomplish the selectionof resistance values. Since FIGS. 1-5 illustrate circuits 10 and 14 in alowering configuration, the hoist contact 50 in the motor circuit 10remains open.

FIG. 2 illustrates the same motor circuit 10 as shown in FIG. 1;however, the arrows indicate the direction of reverse current flowcaused by motor regeneration when a power loss occurs during thelowering of a heavy load. The downward movement of the load will causethe motor to be in an overhauling state in which it will generatesufficient current to keep the series-wound coil 34 of the solenoidenergized and thus prohibit the setting of the series brake. There willalso be sufficient current flowing in the control circuit 14 (FIG. 1) tokeep the UV relay 34 energized, thereby maintaining the UV contact 42 ina closed state. If the weight of the load is greater than the effect ofthe motor's dynamic braking, the load will continue to drop at a ratecontrolled by dynamic braking until the CEMF is reduced to a level thatwill permit the UV contact 42 to open, or until the load reaches theground. The circuits 10 and 14, as shown in FIGS. 1 and 2, have nomethod of preventing regenerated current from the motor circuit 10 fromflowing back into the crane control circuit 14 other than the emergencystop button or "dead man" switch, which require action by the craneoperator.

FIG. 3 illustrates a crane operating circuit of the type generallydescribed by Urbassik in the A.I.S.E. paper of 1997, and generallyindicated by reference numeral 70. Operating circuit 70 employs themotor circuit 10, control circuit 14, power supply 18 as describedabove, and includes a low voltage monitoring relay (LVMR) 74 and acontact 78 or similar circuit opening device operated by the LVMR 74.The contact 78 is connected in series with the undervoltage relay 34 ofthe crane control circuit 14. The LVMR 74 monitors the voltage of a DCbus 82, which provides power to the power supply 18. If the LVMR 74detects a change in the DC bus 82 voltage, the contact 78 is opened,causing the low voltage relay 34 to open the low voltage contact 42. Theopen contact 42 prevents any regenerated current from the motor circuit10 entering the control circuit 14.

FIG. 4 illustrates a crane operating circuit constructed in accordancewith the present invention and generally indicated by reference numeral86. In this illustration, as in FIG. 1, the arrows indicate thedirection of current flow during a normal load lowering operation.Operating circuit 86 employs the motor circuit 10, control circuit 14,power supply 18 as described above, and includes an anti-regenerationcircuit comprised of a current blocking device 90 in the motor circuit10 and a surge suppresser 94 in the control circuit 14. The orientationof the current blocking device 90 in the motor circuit 10 blocks theflow of current produced by an overhauling DC motor during a poweroutage, but does not block the normal flow of current from the DC powersource 18 in the crane control circuit 14. In the preferred embodiment,the current blocking device 90 is a power diode connected electricallyin series with the lowering contact 22 and the motor armature 26 of themotor circuit 10. The current blocking device (power diode) 90 should berated to continuously withstand 200% of the rated motor current for thelargest NEMA size motor in which the anti-regeneration circuit will beinstalled. During a power loss, a high inverse peak voltage is generatedby the de-excitation of the control circuit 14. This high inverse peakvoltage will be placed across the current blocking device 90. It istherefore important to provide a surge suppresser 94 in the controlcircuit 14, which will prevent the high inverse peak voltage fromdamaging the current blocking device 90. It is also important that thesurge suppresser 94 not be actively in the control circuit 14 duringnormal operation, when power is being provided by the DC power source18. Therefore, the surge suppresser 94 consists of two components, anenergy dissipation device 98, which reduces the high inverse peakvoltage to a level which will not damage the current blocking device 90,and a selective activation device 102, which determines when the energydissipation device 98 will be actively in the control circuit 14. In thepreferred embodiment, the energy dissipation device 98 is a dissipationresistor, and the selective activation device 102 is a controlrectifier. The dissipation resistor and control rectifier areelectrically connected in series with one another. The selectiveactivation device 102 is connected in the control circuit 14 such thatnormal current flow from the DC power source 18 will not pass throughthe dissipation device 98, but current produced by the high inverse peakvoltage caused by rapid de-excitation of relay coils in the controlcircuit 14 will pass through the dissipation device 98. The dissipationdevice 98 and selective activation device 102 together are connectedelectrically in parallel with the control circuit 14. The value of thedissipation resistor must be selected such that the current blockingdevice 90 will never be subjected to a peak inverse voltage greater thanit is rated for. Therefore, the value of the dissipation device 98 mustbe selected based on the maximum inverse peak voltage that can beproduced by the de-excitation of relay coils in the control circuit 14for the largest NEMA rating for a motor in which the anti-regenerationcircuit will be installed. As an example, for a NEMA size 5 or 5Acontrol circuit 14: the blocking device 90 can be a Semikron SKN 1500/20rectifier diode or equivalent with a heat sink such as Semikron P 11 orequivalent; the selective activation device 102 can be a Semikron SKN20/16 or equivalent with a heat sink such as Semikron K3-M6 orequivalent; and the dissipation device 98 can be two 150 ohm Ohmite typeresistors having a rating of 200 watts each, connected in series, ortheir equivalent. All of these components are commercially available.

FIG. 5 illustrates the same crane operating circuit 86 as shown in FIG.4, however the arrows indicate the direction of current flow caused bymotor regeneration when a power loss occurs during the lowering of aheavy load. The blocking device 90 of the automatic anti-regenerationcircuit prevents regenerated current produced by the DC motor during apower outage from entering the control circuit 14, but does not preventnormal crane control circuit current flow under all normal operatingconditions. The anti-regeneration circuit, when installed in a craneoperating circuit 86, is active at all times and does not require amonitoring relay 74 or a contact 82 to open the control circuit 14, nordoes it require any action by the crane operator.

The current blocking device 90 and surge suppresser 94 of theanti-regeneration circuit of the present invention can be incorporatedinto a printed circuit board for retrofitting the existing motor andcrane control circuits, 10 and 14, respectively, of an existing craneoperating circuit 12 in the field. The retrofit is accomplished bymaking electrical connections between the circuit board of the existingmotor and crane control circuits, 10 and 14, respectively, and theanti-regeneration circuit. The connections can be made by means ofsimple plug-in connectors or by connecting jumper wires between selectedpoints of the original existing motor and crane control circuits, 10 and14, respectively, and electrical terminals on the anti-regenerationcircuit printed circuit board.

I claim:
 1. An operating circuit for a crane having a DC lifting motorcircuit and a control circuit, said operating circuit comprising:aseries-wound brake activating solenoid; an anti-regeneration circuitincluding a current blocking device electrically in series with saidseries-wound brake activating solenoid and a surge suppresserelectrically in parallel with said control circuit, said currentblocking device preventing a regenerated DC current from flowing in saidcontrol circuit and said surge suppresser protecting said blockingdevice from a reverse DC voltage spike caused by a sudden loss of powerin said control circuit.
 2. The crane operating circuit of claim 1wherein said blocking device is a power diode.
 3. The crane operatingcircuit of claim 2 wherein said power diode is electrically in serieswith an electrical switch controlling a lowering circuit of said cranecontrol circuit.
 4. The crane operating circuit of claim 3 wherein saidpower diode is connected such that current flow in said crane controlcircuit from a DC power source providing operating power to said cranecontrol circuit and the DC lifting motor under normal operatingconditions is not blocked.
 5. The crane operating circuit of claim 3wherein said power diode is connected such that reverse current flow insaid crane control circuit resulting from overhauling of the DC liftingmotor is blocked.
 6. The crane operating circuit of claim 1 wherein saidsurge suppresser comprises a selective activation device and adissipation device electrically connected in series with one another. 7.The crane operating circuit of claim 6 wherein said selective activationdevice is a control rectifier and said dissipation device is a dischargeresistor.
 8. The crane operating circuit of claim 7 wherein said controlrectifier is configured such that said discharge resistor iselectrically active only when said crane control circuit does notreceive power from said DC power source.
 9. A crane operating circuitcomprising:a DC power source for providing power to a DC lifting motorcircuit and a control circuit; a series brake activating circuit foractivating a series brake upon a sudden loss of said power provided bysaid DC power source; a current blocking device for blocking aregenerated current produced by the overhauling of said DC lifting motorresulting from said sudden loss of power, said blocking devicepreventing the regenerated DC current from flowing in said crane controlcircuit and thereby maintaining said series brake activating circuit inan non-activating state; a surge suppresser for protecting said currentblocking device from a high peak reverse voltage caused by the suddenloss of power in the control circuit.
 10. The crane operating circuit ofclaim 9 wherein said series brake activating circuit is a solenoidenergized by current flow in said crane control circuit.
 11. The craneoperating circuit of claim 9 wherein said blocking device is a powerdiode.
 12. The crane operating circuit of claim 11 wherein said powerdiode is electrically in series with an electrical switch supplyingpower to said crane control circuit.
 13. The crane operating circuit ofclaim 12 wherein said power diode is connected such that normal currentflow in said crane control circuit is not blocked, but reverse currentflow in said crane control circuit is blocked.
 14. The crane operatingcircuit of claim 9 wherein said surge suppresser comprises a controlrectifier and discharge resistor electrically in series with one anotherand electrically in parallel with said crane control circuit.
 15. Thecrane operating circuit of claim 14 wherein said control diode isconfigured such that said discharge resistor is electrically active onlywhen said crane control circuit does not receive power from said DCpower source.
 16. An anti-regeneration circuit for retrofitting a DCcrane operating circuit having a motor circuit and a control circuit,said anti-regeneration circuit comprising:a printed circuit board havinga plurality of electrical terminations, each said termination providinga connecting means to particular point of the DC crane operatingcircuit; a current blocking device mounted on said printed circuitboard, said current blocking device having a first end electricallyconnected to a first one of said plurality of electrical terminationsand a second end connected to a second one of said plurality ofelectrical terminations; and a surge suppresser also being mounted onsaid printed circuit board, said surge suppresser having a first endelectrically connected to a third one of said plurality of electricalterminations and a second end electrically connected to a fourth one ofsaid plurality of electrical terminations.
 17. The crane operatingcircuit of claim 16 wherein said blocking device is a power diode havingits anode connected to said first electrical terminal and its cathodeconnected to said second electrical terminal.
 18. The crane operatingcircuit of claim 17 wherein said first electrical terminals is to beconnected to a positive DC connection point of said motor circuit andsaid second electrical terminal is to be connected to a negative DCconnection point of said motor circuit such that said blocking device isconnected electrically in series with a switch controlling a loweringcircuit of said motor circuit.
 19. The crane operating circuit of claim16 wherein said surge suppresser comprises a selective activation deviceand a dissipation device electrically connected in series with oneanother and electrically connected in parallel with said controlcircuit.
 20. The crane operating circuit of claim 19 wherein saidselective activation device is a control rectifier and said dissipationdevice is a discharge resistor, the cathode of said control rectifierbeing connected to said third electrical terminal and the anode of saidcontrol rectifier being connected to one end of said discharge resistor,said fourth electrical terminal to be connected to the other end of saiddischarge resistor, said third terminal being connected to a positive DCconnection point of said control circuit and said fourth electricalterminal to be connected to a negative DC connection point of saidcontrol circuit.